Either Configure saml using configuration as code

or

Configure your saml using ui.

Go to configuration as code menu & copy "securityRealm" section.

Your configuration as code is stored not on disk but in jenkins-jenkins-jcasc-config configmap. Edit it (kubectl edit or kubectl get oyaml + apply) & replace local section with saml copied from configuration as code UI (add proper indents, probably 4).

Done.